The application of a previously developed method for fitting skill acquisition data with hyperbolic functions is shown by means of ft model and actual data. The hyperbolic function yields explicit information on prior practice. The ability to account for prior practice makes it possible to analyse experimental methodology in a more rigorous manner than previously possible. The error that may be inherent in typical procedures can be quantified. The errors are greatest at early stages of learning, smallest when learning is far advanced. From the equations, one can calculate the number of trials required before the error term is below a. specified value.